Vacuum pump control



6, 1967 R. s. WOOLDRIDGE 3,

VACUUM PUMP CONTROL Filed April 23, 1965 INVENTOR ROBERT S. WOOLDRIDGE By W%/ ATTORNEY United States Patent 3,360,185 VACUUM PUMP CONTROL Robert S. Wooldridge, Norristown, Pa., assignor to Sperry Rand Corporation, New York, N.Y., a corporation of Delaware Filed Apr. 23, 1965, Ser. No. 450,434 5 Claims. (Cl. 230-42) ABSTRACT OF THE DISCLOSURE A system for regulating the pressure in a tape loop box for magnetic tape handlers is disclosed. A vacuum pump drive by a motor is used to create a partial vacuum between the bottom of the tape loop and the bottom of the loop box. A pressure sensitive switch is used to sense the vacuum in the loop box and to control the average speed of the vacuum pump motor so as to hold the vacuum in the loop box substantially constant.

This invention relates to tape loop boxes, and more particularly to means for regulating the vacuum in such tape 100:: boxes.

Tape loop boxes wherein loops of magnetic tape are maintained during operation are well known. The patent to Welsh 3,189,291 issued June 15, 1965, and the patent to Weidenhammer et al. 2,792,217 issued May 14, 1957, are but two examples of tape handlers that show vacuum loop boxes of the type with which the present invention is concerned. In general, such loops are provided to permit relatively fast acceleration of the tape. The loops of tape are generally formed by a pressure differential resulting from atmospheric pressure at the top of the tape and a low pressure or vacuum at the bottom of the tape.

The low pressure or vacuum is generally created by a vacuum pump which is driven by a pump motor. Very often, it is desirable to use relatively inexpensive series motors to drive the vacuum pumps. When the source of voltage driving the motor is subjected to wide variations, as is often the case, the motor is driven at different speeds therebv causing changes in the vacuum within the loop box. This, in turn, causes the tension on the tape to vary resulting in irregular operation. For example, as the tension on the tape varies, the rates of acceleration of the tape also vary causing faulty operation of the read-write circuits.

The length of the loop of tape is often controlled by the operation of a vacuum switch. Such switches are re "sponsive to open or close dependent upon the pressure be tween the tape and the bottom of the loop box. Many times, switches having special characteristics and designed to operate at selected pressures are necessary to assure uniformity of the length of the loop and tension of the tape.

When tape transport units are shipped from their places of manufacture to their places of operation, the environmental conditions under which they operate vary widely. This is especially true where ditierences in atmospheric pressure are encountered. Because of the variations in the atmospheric pressure is different environmental conditions, substantially identical loop boxes will cause different loop lengths and tensions in the tape.

It is an object of this invention to provide a novel pressure control system.

It is a further object of this invention to provide an improved vacuum regulating device for controlling pressure in a tape loop box.

It is still a further object of this invention to provide improved means for maintaining tension on a loop relatively constant during operation.

3,360,185 Patented Dec. 26, 1967 It is still a further object of this invention to provide an improved means for minimizing the eiiects of variable acceleration and variations in skew in a tape resulting from the tension on the tape varying during operation.

It is still a further object of this invention to provide an improved means for varying the tension on a tape to adjust for diiierences in atmospheric pressure in the environments in which the tape is operated.

It is still a further object of this invention to provide an improved pressure control system wherein a vacuum pump may be subjected to Widely varying input driving voltage.

It is still a further object of this invention to provide an improved pressure control system for a tape loop box wherein the vacuum switches employed may have widely varying characteristics.

In accordance with the present invention, means are provided for regulating pressure in a tape loop box. A vacuum pump, driven by a motor, is used to create a relatively low pressure or vacuum between the tape and the bottom of the box. A vacuum switch, responsive to the pressure in the loop box, connects a source of voltage to the motor to control the speed of the motor. An adjustable valve is provided to control the pressure at which the vacuum switch opens and closes. The voltage applied to the motor is caused to remain at a relatively constant level and drives the pumpto maintain a relatively constant desired pressure in the loop box.

Other objects and advantages of the present invention will be apparent from a reading of the following specification and claims, in conjunction with the sole figure of the drawing which illustrates one embodiment of the present invention.

Referring to the drawing, a loop box 10, which corresponds to the one of the tape columns 24 or 26 of Patent 2,792,217, for example, includes a loop of magnetic tape 12. A low pressure of vacuum is created in the area 14 between the bottom surface of the tape 12 and the sides and bottom of the loop box 10.

The low pressure area is created by mans of a vacuum pump 16 driven by a DC. series motor 18. A source of voltage is applied from terminal 20 to drive the motor 18 through appropriate circuitry, as will be described. The size or capacity of the pump 16, while not critical, is selected so that when motor 18 is full on, pump 16 will generate the necessary vacuum to pull the tape loop 12 down to its desired operating position in loop box 10 when the tape handler is operating under the designed worse case condition of atmospheric pressure and line voltage variation.

A silicon controlled rectifier 22, including an anode electrode 21, a cathode electrode 23 and a gate electrode 26, is provided to control the current through the winding 24 of the motor 18.

The silicon controlled rectifier 22 is appropriately :biased by resistors 25, 27 and 29 so that it is normally cut-off in the absence of an applied gating signal to its gate electrode. A voltage from a vacuum switch unit 30 is applied to make the silicon controlled rectifier 22 conducting. When a gating signal is applied to the gate electrode 26 of the rectifier 22, it will become conducting causing current to flow from terminal 20, through the winding 24 of the motor, through the anode and cathode of the rectifier 22, to a point of reference potential, designated as ground.

Silicon controlled rectifiers are well known to those skilled in the art. Their main advantage is that they are relatively inexpensive and are capable of drawing heavy current. Such devices act as normal rectifiers, i.e., they include an anode and a cathode, but also include a third electrode for receiving a gating signal. Silicon controlled rectifiers are normally held non-conducting until an appropriate gate signal is applied to its gate electrode. Once a silicon rectifier becomes conducting, it generally remains conducting, even though the control signal applied to the gate electrode is discontinued.

Consequently, if a silicon controlled rectifier is to be shut off after the gating signal is discontinued, some appropriate signal must be applied to the rectifier to cut it off, In the embodiment illustrated in the present invention, the voltage applied at the terminal is an unfiltered, full wave, rectified DC. voltage, as illustrated by a wave form 32. It is noted that while the wave form 32 is positive for a full half cycle, it also goes slightly negative for a short time. This is illustrated by the small portions of the DC. signal falling slightly below the reference level, designated as zero. With the type of signal 32 applied to the silicon controlled rectifier, the rectifier is cut off by the slightly negative portions of the signal 32. The use of such silicon controlled rectifiers, as well as the use of unfiltered DC. voltage to shut them off, are well known to those skilled in the art, and are only incidentally related to the present invention.

A diode is provided across the motor 18 to receive the high current resulting from the inductive reactance of the motor when the motor is suddenly turned off. This diode is also only incidentally related to the present invention.

The silicon controlled rectifier 22 is made conducting by the application of a gating signal from the vacuum switch unit 30 to permit current to flow through the motor in a manner to be described.

The vacuum switch unit 30 includes a flexible diaphragm 34 which divides the unit into two chambers 36 and 38. The chamber 36 is exposed to the atmosphere and the chamber 38 is connected through conduits 4t and 42 to the low pressure or vacuum area 14 of the tape loop box. The chamber 38 is also connected to the atmosphere through the conduit 40, a conduit 44 and an adjustable valve 46.

Included within the chamber 38 is a vacuum switch 48 adapted to be normally closed and to be open when a predetermined pressure is present within. the chamber 38. The switch includes a movable contact arm 49 which closes or opens to connect a source of gating voltage from a terminal 50 through the resistor 29 to the gate electrode 26 of the silicon controlled rectifier 22'.

The adjustable valve 46 provides a leakage path for the pressure from the low pressure area 14 while the conduit 42 acts as a restrictor whereby the valve 46 and the conduit 42 produce an adjustable pressure drop between the switch 30 and the pressure in chamber 14. The pressure at which the switch 48 is actuated is controlled by the position of the adjustable valve 46. If different atmospheric pressures are involved because of different environments for different tape transport units of the same type, different switches all having the same set response characteristic may still be used by simply adjusting the valve 46. The adjustable valve 46 eliminates the need for special switches for different environmental conditions.

The pressure level in the area 14 is dependent upon the degree of tension to be applied to the tape 12. In most situations, it is desirable to maintain the tension on the tape 12 relatively constant. The tension on the tape is dependent upon the pressure difiFerence between the pressure of the atmosphere above the tape and the pressure in the area 14.

Let us first consider a situation in which the pressure in the low pressure area 14 drops below some predetermined desired level. The differential pressure or tension on the tape 12 will then increase. When this occurs, it is desirable to bring the pressure within the area 14 to a higher level thereby decreasing the differential pressure on the tape loop. This may be done by decreasing the speed of the motor 18 which, in turn, slows down the operation of the vacuum pump 16.

When the pressure in the area 14 drops, the movable contact 49 of the vacuum switch 48 is held open thereby disconnecting the source of voltage from the terminal 50 to the gate electrode 26 of the silicon controlled rectifier 22. When this occurs, the silicon controlled rectifier is held non-conducting since no gating signals are present to drive it to conduction. When the silicon controlled rectifier is non-conducting, no current flows through the motor 18 causing it to slow down. The slowing down of the motor 18 causes the vacuum pump'to operate at a slower speed thereby permitting the pressure in the area 14 to increase. Consequently the differential pressure across the tape decreases to decrease the tension on the tape to its desired degree.

Consider now a situation in which the degree of pressure in the area 14 rises thereby causing the differential pressure and the tension on the tape 12 to decrease. In this situation, it is desirable to speed up the motor 18 and the vacuum pump 16 to lower the pressure or produce more vacuum in the area 14. This is accomplished by permitting the movable contact arm 49 of the vacuum switch 48 to remain closed thereby permitting the voltage from the terminal 50 to be applied to the gate electrode 26. This causes the silicon controlled rectifier 22 to become conducting to increase the driving current through the motor.

Because the movable arm 49 is closed to maintain the rectifier 22 conducting, the motor 18 is caused to speed up. This in turn increases the speed of the vacuum pump 16 thereby creating a lower pressure in the vacuum area 14. Then tension on the tape therefore increases.

The present invention has provided a relatively simple means for maintaining tension of a tape relatively constant. This is accomplished by standard parts, such as vacuum switches of non-critical types, and may be used in widely varying environments involving different atmospheric pressures. Also, the use of relatively inexpensive motors is possible. The input voltage to drive the motor may also vary over wide ranges because of the compensating features of the present invention.

While such terms as atmospheric pressure and vacuum have been employed, it should be understood that these are only relative terms. The term vacuum is to be interpreted as not necessarily being a pure vacuum but simply a source of relatively low pressure with respect to another source of pressure, such as atmosphere.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In combination, a tape loop control box, a tape loop formed in said control box so that one surface of said tape loop is exposed to the atmosphere and the other surface thereof forms a vacuum chamber with said control box, means for regulating the vacuum in the vacuum chamber of said box comprising vacuum producing means for producing vacuum in said box, control means for controlling the operation of said vacuum producing means to control the amount of vacuum in said box, and switching means responsive to the vacuum in said box to actuate said control means, means communicating with both the switching means and the control box for adjusting the vacuum applied to said switching means to maintain a desired vacuum in said loop box.

2. In combination a tape loop control box, a tape loop formed in said control box so that one surface of said tape loop is exposed to the atmosphere and the other surface thereof forms a vacuum chamber with said control box, means for regulating the vacuum in the vacuum chamber of said box comprising a pump for producing a vacuum in said box, a motor for driving said pump, a signal source to operate said motor, switching means responsive to the vacuum in said loop box to connect said signal source to operate said motor, and means communicating with both the switching means and the control boxes for adjusting the vacuum applied to said switching means to maintain a desired length of tape loop in said loop box.

3. In combination with a tape loop box wherein one surface of said tape is exposed to the atmosphere and the other surface forms a vacuum chamber with said loop box, the length of said tape loop being determined by the pressure differential between the pressure of said atmosphere and the pressure within said vacuum chamber, means for controlling the length of tape loop in said loop box comprising a motor for producing a vacuum within said vacuum chamber to maintain said tape loop at a predetermined length, a source of current for driving said motor, a silicon controlled rectifier circuit to control the current through said motor, a switch responsive to the vacuum in said vacuum chamber to control the operation of said silicon controlled rectifier, an adjustable valve for adjusting the vacuum applied to said switch whereby a predetermined length of loop may be maintained in said loop box for diiferent atmospheric pressures.

4. In combination with a loop control box wherein one surface of said tape is exposed to the atmosphere and the other surface forms a vacuum chamber with the sides and bottom of said loop box, the length of said tape loop being determined by the pressure differential between the pressure of said atmosphere and the vacuum within said vacuum chamber, means for controlling the length of tape loop in said loop box comprising a pump for producing a predetermined vacuum within said vacuum chamber to maintain said tape loop at a predetermined length, a motor for driving said pump, the vacuum produced by said pump being dependent upon the speed of said motor, a source of voltage for supplying current to drive said motor, a silicon controlled rectifier circuit adapted to be switched between conducting and non-conducting states to control the current through said motor, a switch responsive to the vacuum in said vacuum chamber to cause said silicon controlled rectifier to be switched from a non-conducting to a conducting state, means for adjusting the vacuum applied to said switch whereby a set predetermined vacuum actuates said switch despite difierent pressures in said vacuum chamber or variations in said pressure differential whereby a predetermined length of loop may be maintained for dilferent atmospheric pressures.

5. In combination with a tape loop control box wherein one surface of said tape is exposed to the atmosphere and the other surface forms a vacuum chamber with the sides and bottom of said loop box, the length of said tape loop being determined by the pressure differential between the pressure of said atmosphere and the vacuum within said vacuum chamber, means for controlling the length of tape loop in said loop box comprising a motor for producing a predetermined vacuum within said vacuum chamber to maintain said tape loop at a predetermined length, a source of voltage for supplying current to drive said motor, a silicon controlled rectifier circuit adapted to be switched between conducting and non-conclucting states to control the current through said motor, a switch connected in a first path to said vacuum chamber and responsive to the vacuum in said vacuum chamber to cause said silicon controlled rectifier to be switched from a nonconducting to a conducting state, and an adjustable valve connected in a second path divergent to said first path for adjusting the vacuum applied to said switch whereby a set predetermined vacuum actuates said switch despite different pressures in said vacuum chamber or variations in said pressure differential whereby a predetermined length of loop may be maintained for difierent atmospheric pressures.

References Cited UNITED STATES PATENTS 440,375 1 1/1890 Smith 23012 862,696 8/1907 Batcheller 230-12 1,073,276 9/1913 Mohr 230-17 1,132,777 3/1915 Jennings 230-12 1,347,194 7/ 1920 Washington 230-17 3,151,796 10/1964 Lipschutz 22697 3,156,423 11/1964 Potter 242-55.125 3,258,183 6/1966 Weissbach 226 DONLEY I. STOCKING, Primary Examiner. L. V. EFNER, W. L. FREEH, Assistant Examiners. 

1. IN COMBINATION, A TAPE LOOP CONTROL BOX, A TAPE LOOP FORMED IN SAID CONTROL BOX SO THAT ONE SURFACE OF SAID TAPE LOOP IS EXPOSED TO THE ATMOSPHERE AND THE OTHER SURFACE THEREOF FORMS A VACCUM CHAMBER WITH SAID CONTROL BOX, MEANS FOR REGULATING THE VACUUM IN THE VACUUM CHAMBER OF SAID BOX COMPRISING VACUUM PRODUCING MEANS FOR PRODUCING VACUUM IN SAID BOX, CONTROL MEANS FOR CONTROLLING THE OPERATION OF SAID VACUUM PRODUCING MEANS TO CONTROL THE AMOUNT OF VACUUM IN SAID BOX, AND SWITCHING MEANS RESPONSIVE TO THE VACUUM IN SAID BOX TO ACTUATE SAID CONTROL, MEANS COMMUNICATING WITH BOTH THE SWITCHING MEANS AND THE CONTROL BOX FOR ADJUSTING THE VACCUM APPLIED TO SAID SWITCHING MEANS TO MAINTAIN A DESIRED VACUUM IN SAID LOOP BOX. 